The correct option is A: A fort was established and many were killed by disease.
The Lewis and Clark expedition built a fort named "Fort Mandan" in 1804-1805. It was located on the Missouri river.
In order to protect themselves and the people from the harsh winter, Lewis and Clark, with the help of Native Americans, spent much of their time building the fort. They were supposed to establish friendly relationships with native Americans and prepare them for the arrival of the United States traders to the region. They were also supposed to claim the United States territorial sovereignty over the land that had been occupated by Native Americans for thousands of years. But the tribes and the Europeans Americans differed in their concepts of "property".